1. Field of the Invention
The present invention relates to systems and methods for microwave imaging and sounding, and in particular to a system and method for microwave imaging and sounding with large bandwidth radiometer reflector antennas.
2. Description of the Related Art
Conventional satellite microwave imaging and sounding systems typically use a single antenna with a multi-frequency horn to collect radiometric information over multiple frequency bands and a polarizer to separate the vertical and horizontal signal components.
Prior art radiometer reflector antenna designs for microwave imaging and sounding are fed by feed horns or fed by a beam waveguide system. Furthermore, the beam waveguide system in such antennas is designed with reflective surfaces and/or Frequency Selective Surfaces (FSSs) serially arranged in a single array. These systems typically operate from 10 to 90 GHz frequency range and occupy a large portion of the available spacecraft volume. Such prior art designs have been used on devices such as the advanced microwave sounding unit (AMSU) and the microwave humidity sounder (MHS).
Rather than using FSSs, other designs may utilize multi-frequency feeds in combination with diplexers to separate received signals into component frequencies. However, diplexers are bulky and result in inferior electrical performance.
To meet higher bandwidth requirements, there is a need to extend the operating frequency of transceivers to a range used on such satellites from 6 to 200 GHz. This results in a larger, more complicated, and costly design. Moreover, in many cases such performance improvements cannot be realized within the limited available physical envelope of the spacecraft employing prior art designs.
Furthermore, in the design of spacecraft, there are certain constant desirable objectives, which tend to vary only in emphasis for any particular application. These include reducing the time required to build components, improving the manufacturability, and of course, reducing component cost. A versatile design, easily modified through the alteration of simple parameters furthers these objectives.
There is a need in the art for systems and methods of microwave imaging and sounding which efficiently provide a broader frequency range in a compact form. There is a further need for systems and methods which are easily modified, less costly and lighter.
The present invention satisfies these needs.
To address the requirements described above, the present invention discloses an apparatus and method for microwave imaging and sounding with large bandwidth radiometer reflector antennas in a compact form.
The apparatus comprises a main reflector for reflecting a beam, a polarizer for polarizing the reflected beam into a first and second polarized beam, a first FSS reflecting a first selected frequency of the first polarized beam, a second FSS reflecting a second selected frequency of the second polarized beam, a first feed receiving the first selected frequency and a second feed receiving the second selected frequency.
The method comprises reflecting a beam, polarizing the reflected beam into a first and second polarized beam, reflecting a first selected frequency of the first polarized beam, reflecting a second selected frequency of the second polarized beam, receiving the first selected frequency and receiving the second selected frequency.
This invention is directly applicable to the design of radiometer reflector antennas, which utilize extremely large bandwidths and require packaging in a compact volume, as is often necessary in spacecraft applications. The invention allows the polarized components of an incoming beam to be directed to and used by the respective frequency specific feed horns in parallel, rather than in series.
One embodiment allows a substantial reduction in the packaging envelope, which is typically required to accommodate a sensor suite covering a broad frequency range. In addition to a more compact and efficient packaging arrangement, the present invention provides enhanced electrical performance by providing frequency-specific dedicated feeds, as opposed to multi-frequency feeds with bulky and lossy diplexers. This design allows simple control of frequency selection by replacing the FSS without impacting any other mechanical parameters of the design. Through a reflection and transmission filtering process the present invention provides versatile and convenient frequency selection and horn orientations.
In one embodiment, the efficient utilization of space is accomplished by rotating the surface geometry about one of the surface focal points, co-located at the main reflector focal point, and locating the respective feed horn at a second surface focal point.
Another embodiment provides a system and method for collecting the same data as prior art systems and methods utilizing one antenna reflector with a series of nested FSSs. However, the present invention produces a larger frequency bandwidth within a much tighter physical envelope by utilizing parallel groups of FSSs. The parallel groups of FSSs are enabled through the use of a flat plate polarizer which effectively creates two separated polarized component focal points of the main reflector. The parallel FSS groups are positioned relative to each of these main reflector focal points.
The foregoing allows collection of data as prior art systems and methods utilizing one antenna reflector but over a broader frequency range and in a more compact and versatile design.